Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
About "Mystery Creeks": I love small streams, but some of my favorite little trout streams are too small and too fragile to publicize here. If you recognize one of these, you already understand why I'm keeping it a secret. These are the kinds of places that lose a little bit of their charm if you see someone else's week-old footprint, and I don't want to do that to them.
This specimen is 14 mm. Technically this is the subspecies T. h. hecuba. The Cascades, Sierras and further West is where the other subspecies, T. h. pacifica is found. The Great Basin seems to have formed a barrier preventing any overlap in their distribution.